The Internet and the intranet in the past offer best-effort type services in most cases. The TCP (Transmission Control Protocol) is mainly used as a protocol. A terminal that performs communication using the TCP establishes a TCP connection through sequence processing called 3-way handshake between the terminal and a partner terminal and then performs transmission and reception of a TCP data packet. To realize reliable communication in a network in which a band and a delay time are unknown, the TCP includes a congestion control mechanism for controlling congestion using an acknowledgement (ACK) and a sequence number. The terminal that performs communication using the TCP autonomously controls a transmission amount of TCP packets per a unit time. Such a technology is disclosed in Non Patent Literature 1.
A broadband router that connects a local network to an external network such as the Internet includes a filtering mechanism for blocking an unauthorized packet transmitted from the external network. As a highly functional filtering method, there is an SPI (stateful packet inspection) function for dynamically setting filter conditions automatically to allow only truly-necessary packets to pass after communication is started. A router can prevent, by following a state transition of the TCP using the SPI function, intrusion of an unauthorized packet, which imitates a TCP packet, into an internal network. Such a technology is disclosed in Non Patent Literature 2.
In recent years, in mobile bodies such as a car, a train, an airplane, and a ship, it is also demanded to connect local networks on the inside of the mobile bodies to an external network such as the Internet using various radio communication systems. A part of Internet services is realized in a bus, a train, and an airplane by using the technologies disclosed in Non-Patent Literatures 1 and 2. Various apparatuses unrelated to networks in the past start to include functions assumed to be used in environments in which the networks can be used. Communication applications for performing communication using the Internet connection increase. Therefore, the communication applications range from those for business and control to those for entertainment. Kinds of communication considered to have various importance degrees and priority degrees are carried out between local networks on the inside of mobile bodies and the Internet on the outside.
As a form of use of networks in such mobile bodies, downlink traffic, which is data transfer from the external network to the local networks in the mobile bodies, such as download of map information contents or the like is more often used than uplink traffic, which is data transfer from the local networks in the mobile bodies to the external network. TCP/IP is used as a communication protocol in most of these kinds of communication. When a terminal detects a loss of a communication packet, the terminal autonomously determines that abnormality of a network occurs and performs control for limiting a packet transmission amount per unit time to enable communication.